The present disclosure relates to a power supply system that includes a fuel cell as its energy source and has not only high energy density but also high power density.
In recent years, portable electronic apparatus such as cellular phones, notebook personal computers, digital cameras, and camcorders have a tendency that its power consumption increases along with enhancement in its functions and increase in the number of its functions. As the power supplies of these pieces of portable electronic apparatus, small primary batteries and secondary batteries are used in general.
Parameters showing the cell characteristics include the energy density and the power density. The energy density refers to the electric energy that can be supplied per unit mass or unit volume of the cell. The power density refers to the power output per unit mass or unit volume of the cell. The cell used in the portable electronic apparatus is required to have enhanced energy density and power density so that the cell can be compatible with further enhancement in the functions of the electronic apparatus and further increase in the number of functions of the electronic apparatus.
For example, a lithium-ion secondary battery, which is widely spread as the power supply of portable electronic apparatus currently, has an excellent characteristic of high power density. Furthermore, the energy density thereof is also comparatively high, and the volumetric energy density thereof reaches 400 Wh/L or higher. However, further great enhancement in the energy density of the lithium-ion secondary battery can not be expected unless materials thereof are greatly changed.
Therefore, a fuel cell is expected as the next-generation power supply for portable electronic apparatus, in order to keep compatibility with the portable electronic apparatus, for which the progression of further increase in the number of functions and the power consumption in the future is expected.
In the fuel cell, a fuel is supplied to the anode side and the fuel is oxidized, and air or oxygen is supplied to the cathode side and oxygen is reduced. In the whole fuel cell, reaction of oxidation of the fuel by oxygen occurs. As a result, the chemical energy of the fuel is efficiently converted to electric energy and extracted. Therefore, if refueling is continued, using the fuel cell as the power supply can be continued without charge, unless failure of the fuel cell occurs.
Experimental manufacturing of various fuel cells has been already performed, and part of the fuel cells has been put into practical use. Of these fuel cells, the fuel cells having the highest possibility of the use as the power supply for portable electronic apparatus are polymer electrolyte fuel cells (PEFC) having a proton-conducting polymer membrane as its electrolyte. Of the PEFCs, a direct methanol fuel cell (DMFC), in which methanol is supplied to the anode as the fuel without being reformed, has the highest possibility.
In the DMFC, the methanol as the fuel is supplied to the anode side as a low-concentration or high-concentration aqueous solution in general, and is oxidized to carbon dioxide in a catalyst layer on the anode side as represented by the following equation (1).Anode: CH3OH+H2O→CO2+6H++6e−  (1)
The hydrogen ions generated at this time pass through the proton-conducting polymer electrolyte membrane sandwiched between the anode and the cathode and move to the cathode side. The hydrogen ions react with oxygen in a catalyst layer on the cathode side as represented by the following equation (2), so that water is produced.Cathode: 6H++(3/2)O2+6e−→3H2O  (2)
The reaction occurring in the whole DMFC is represented by the following reaction equation (3), which results from the synthesis of equation (1) and equation (2).Whole DMFC: CH3OH+(3/2)O2→CO2+2H2O  (3)
It is expected that the volumetric energy density of the DMFC can realize magnitude greater than that of the volumetric energy density of the lithium-ion secondary battery by a factor of several times. However, as one of the problems of the DMFC, low power density is cited. Therefore, if it is attempted to generate the power for operating portable electronic apparatus by the fuel cell solely, there is concern that the size of the fuel cell becomes too large and the fuel cell can not be incorporated in the portable electronic apparatus as a result.
In addition, although the fuel cell is expected to have high energy density in theory, the fuel conversion efficiency (the ratio of the electric energy that can be actually extracted from the fuel to the theoretical value) and the power output are affected by the power generation current and the power generation voltage. Therefore, there is concern that the energy density that can be realized when a load is actually driven (hereinafter, referred to as the effective energy density) is significantly lowered. This point will be described below.
(a) of FIG. 5 is a graph showing a general current-voltage characteristic at the time of the power generation of the DMFC. Because polarization is large in the DMFC, the power generation voltage gradually decreases as the power generation current increases as shown in (a) of FIG. 5.
(b) of FIG. 5 is a graph showing the relationship among the power output, the fuel conversion efficiency, and the power generation voltage of the DMFC. The power output from the DMFC depends on the product of the power generation voltage and the power generation current. Therefore, the power output is low in the region in which the power generation voltage is high but the power generation current is small and in the region in which the power generation current is too large and thus the power generation voltage is lowered due to polarization. The power output takes the maximum value in the region intermediate between these regions. Furthermore, the fuel conversion efficiency of the DMFC is lowered due to methanol crossover and so on in the region in which the power generation current is small, and is lowered because of energy loss due to heat generation attributed to polarization in the region in which the power generation current is large. Therefore, the fuel conversion efficiency is the highest in the region intermediate between these regions. The methanol crossover refers to a phenomenon that the methanol moves from the anode side to the cathode side through the electrolyte membrane.
In order to enhance the effective energy density of the DMFC, it is desirable to cause the power generation of the DMFC in the region in which the fuel conversion efficiency is the highest (around the power generation voltage indicated by the dashed line in (b) of FIG. 5) and efficiently convert the chemical energy of the fuel to electric energy. However, the power consumption of portable electronic apparatus or the like as a load sharply changes with time, and thus the power output of the fuel cell also greatly increases and decreases correspondingly. Therefore, if the DMFC and the load are simply connected, the time zone during which the power generation of the DMFC can be caused in the region in which the fuel conversion efficiency is the highest hardly exists. Accordingly, the fuel conversion efficiency of the DMFC is significantly lowered and the electric energy that can be actually extracted from the DMFC is significantly decreased. As a result, there is fear that the effective energy density of the DMFC is lowered to a level equal to or lower than that of existing lithium-ion batteries.
Therefore, in order to enhance the effective energy density of the DMFC to a value close to the theoretical value, it is essential to provide contrivance for causing the DMFC to invariably perform the power generation in the region in which the fuel conversion efficiency is the highest irrespective of change in the power consumption of the load, such as contrivance for making a situation in which the DMFC can operate in such a manner as to keep constant power generation voltage, constant power generation current, or constant power generation output.
In order to solve these problems, Japanese Patent Laid-open No. Hei 10-40931 (Pages 2 and 7, FIGS. 1 and 4) and so on propose a hybrid power supply system in which a fuel cell and a secondary battery are connected to a load in parallel and at least one of the fuel cell and the secondary battery supplies power to the load. Japanese Patent Laid-open No. Hei 10-40931 includes the following description. Specifically, if such a configuration is employed, when the load is smaller than a predetermined amount and the power output of the fuel cell has a margin, driving of the load and charge of the secondary battery can be performed by the fuel cell. Furthermore, when the load is increased, the load can be driven by both of the fuel cell and the secondary battery. Thus, the capacity of each of the respective cells can be suppressed, and hence increase in the size of the power supply system can be suppressed.
FIG. 6 is a graph for explaining part of the operation of the above-described power supply system based on the current-voltage characteristics of the fuel cell and the secondary battery, shown in Japanese Patent Laid-open No. Hei 10-40931. The operating voltages of the fuel cell and the secondary battery, shown in FIG. 6, are each not the operating voltage of a single cell but the operating voltage of a cell stack obtained by connecting plural cells in series.
In the fuel cell, the power generation voltage decreases due to polarization as the power generation current increases as described above by taking the DMFC as an example. On the other hand, in the secondary battery such as a lithium-ion secondary battery, the region in which the discharge voltage does not change so greatly although the discharge current changes exists over a considerable range. The magnitude of the discharge voltage Vc at this time changes depending on the charge state. To drive the load, the fuel cell and the secondary battery need to be operated in the region in which the drive voltage equal to or higher than the minimum drive voltage of the load can be kept.
If the drive voltage when the load is small is defined as V1(>Vc), the currents supplied from the fuel cell and the secondary battery at this time are obtained as If1 and Ir1, respectively, from FIG. 6. Because If1>>Ir1, most part of the power in this case is supplied from the fuel cell. On the other hand, if the drive voltage when the load is large is defined as V2 (<Vc), the currents supplied from the fuel cell and the secondary battery at this time are obtained as If2 and Ir1, respectively, from FIG. 6. Because If2<Ir2, the power supplied from the secondary battery is higher than the power supplied from the fuel cell in this case.
While the load is increased and the drive voltage is decreased from V1 to V2 across Vc, the power generation current from the fuel cell only increases from If1 to If2. In contrast, the discharge current from the secondary battery greatly increases from Ir1 to Ir2. This shows that most part of the power consumption increased in this period is supplied from the secondary battery. Furthermore, if this increase in the power consumption is borne by only the fuel cell, the power generation voltage of the fuel cell is decreased to a level lower than the minimum drive voltage of the load as is apparent from FIG. 6. This shows that, if the power supply is formed with only the fuel cell, increase in size of the fuel cell is needed so that the power generation voltage equal to or higher than the minimum drive voltage can be kept, and that connecting the secondary battery excellent in the power density in parallel to the fuel cell allows size reduction of the fuel cell and hence size reduction of the whole power supply system.
Furthermore, Japanese Patent Laid-open No. 2003-333708 (Pages 3 to 5, FIGS. 1 and 2, Table 1) proposes a hybrid energy system. In this energy system, a converter (voltage converter) such as a DC/DC converter is connected in series to a fuel cell, and the equilibrium between the output voltage of the converter and the voltage of an energy storage device such as a secondary battery is permitted. In addition, the whole of the fuel cell and the converter is connected to the energy storage device and a load in parallel.
Table 1 shows the summary of main operating modes 1 to 4 in this hybrid energy system. In the table, SOC denotes the energy accumulation state of the energy storage device. SOCU denotes a predetermined high-accumulation state. SOCL denotes a predetermined low-accumulation state. For example, SOCU is the state in which energy of about 70 to 90% of the maximum accumulation amount is accumulated. SOCL is the state in which energy of about 20 to 50% is accumulated. PFCOPT denotes the power output when the fuel cell operates with the optimum efficiency. PREQ denotes the power output amount required by the load. In addition, E storage device is the abbreviation of the energy storage device.
TABLE 1energyaccumulationloadstoragestateconditiondevicefuel cellload driving1SOC >dischargenot operatedE storage deviceSOCU2SOCU >PREQ >dischargepowerE storage device +SOC > SOCLPFCOPTgeneration withfuel cell3PREQ <chargePFCOPT withfuel cellPFCOPToptimumefficiency4SOCL > SOCcharge atpowerfuel cellmaximumgeneration with highspeedpoweroutput
The above-described hybrid energy system operates based on plural operation principles. First, as long as the energy accumulation state SOC of the energy storage device is larger than the high-accumulation state SOCU (SOC>SOCU), the energy storage device supplies all of the power to the load and the fuel cell does not operate (operating mode 1). Second, if SOC is smaller than the high-accumulation state SOCU and larger than the low-accumulation state SOCL (SOCU>SOC>SOCL), the fuel cell operates with the optimum efficiency and generates power of PFCOPT. If the power output amount PREQ required by the load exceeds PFCOPT (PREQ>PFCOPT), the energy storage device supplies the deficiency of the power (operating mode 2). On the other hand, if PREQ is smaller than PFCOPT (PREQ<PFCOPT)) excess power is accumulated in the energy storage device (operating mode 3). Third, if SOC is smaller than the low-accumulation state SOCL (SOCL>SOC), the fuel cell performs power generation of high power output to thereby supply all of the power to the load and accumulate power in the energy storage device at the maximum speed (operating mode 4).
A characteristic of this system is that, as seen in operating mode 1, the energy storage device is kept at the high-accumulation state as much as possible and not the fuel cell but the energy storage device responds to the energy requirement by the load as long as sufficient energy is accumulated in the energy storage device. Japanese Patent Laid-open No. 2003-333708 includes the following description. Specifically, if such a configuration is employed, the fuel cell can be operated with the optimum efficiency irrespective of change in the load, and the size of the fuel cell can be reduced. Furthermore, comparatively rapid response to change in the energy requirement by the load is permitted.
As is apparent from the description made with FIG. 6, in the power supply system of Japanese Patent Laid-open No. 10-40931, the characteristics of the system depend completely on the characteristics of the fuel cell and secondary battery that are used. For example, the discharge current from the secondary battery greatly increases around the point at which the drive voltage is decreased to Vc or lower, and this Vc depends on the current-voltage characteristic of the secondary battery. The magnitude of Vc changes depending on the charge state of the secondary battery, and thus the voltage corresponding to the start of the increase in the discharge current from the secondary battery changes depending on the charge state of the secondary battery.
Furthermore, in order for the fuel cell and the secondary battery to effectively exert the respective functions in this power supply system, the current-voltage curves of the respective cells need to intersect with each other in an appropriate region as shown in FIG. 6. In addition, in order to charge the secondary battery by the power output of the fuel cell, the power generation voltage of the fuel cell must be equal to or higher than the charge voltage of the secondary battery. The charge voltage of the secondary battery is higher than the open terminal voltage Vro thereof, and the power generation voltage of the fuel cell is lower than the open terminal voltage Vfo thereof. Therefore, the open terminal voltage Vfo of the fuel cell needs to be at least higher than the open terminal voltage Vro of the secondary battery.
As described above, in a simple power supply system in which a fuel cell stack and a secondary battery stack are merely connected in parallel like the power supply system of Japanese Patent Laid-open No. Hei 10-40931, the characteristics of the fuel cell and the characteristics of the secondary battery restrict each other, which causes a limit to enhancement in the system performance and stability.
As for the energy system of Japanese Patent Laid-open No. 2003-333708, there is ample room to enhance the system performance because the fuel cell is connected to the energy storage device and the load via the converter (voltage converter) such as the DC/DC converter. However, as is apparent from FIG. 2 in Japanese Patent Laid-open No. 2003-333708, the control step is complicated and the size of the control system becomes larger, which leads to higher cost.